Golden Eggsteroid #12 (E-12) is located on level 5-12 of Angry Birds Space Red Planet. Zoom out and you will see the Rover on the extreme southeast of the level. Use the Lazer bird to reach the Rover by flinging south of the planets. Once you hit the egg you’ll immediately be transported via a wormhole to the level, which pays homage to the Sojourner Rover. One strategy 3-starring this level is to fire Orange birds just below the small asteroids to fling them toward the piggies and crystals above, maximizing your score!
*Note: Previously known as E-R2 (Sojourner), the level number was changed to E-12 with the 2.0.0 Beak Impact Update.
If you need help, the video below shows the location of the Golden Rover and a strategy for passing the level with 3 stars.